Conventional beam forming systems are often cumbersome to manufacture. In particular, conventional beam forming antenna arrays require complicated feed structures and phase-shifters that are impractical to be implemented in a semiconductor-based design due to its cost, power consumption and deficiency in electrical characteristics such as insertion loss and quantization noise levels. In addition, such beam forming arrays make digital signal processing techniques cumbersome as the operating frequency is increased. In addition, at the higher data rates enabled by high frequency operation, multipath fading and cross-interference becomes a serious issue. Adaptive beam forming techniques are known to combat these problems. But adaptive beam forming for transmission at 10 GHz or higher frequencies requires massively parallel utilization of A/D and D/A converters.
To avoid the problems in the prior art, U.S. Pat. No. 6,885,344 discloses a beam forming antenna system that is compatible with semiconductor processing techniques. For example, FIG. 1 illustrates a beam forming antenna system 100. For illustration clarity, only a single antenna 160 is illustrated. Any suitable topology may be used for the antennas, such as patches, dipoles, or monopoles. For example, antenna 160 may comprise a T-shaped dipole antenna formed using conventional semiconductor processing techniques. Antenna 160 is excited using vias 110 that extend through insulating layers 105 and through a ground plane 120 to driving transistors formed on an active layer 130 separated from a substrate 150 by insulating layer 105. Two elements 101 may be excited by switching layer 130 to form a T-shaped dipole pair 160. To provide polarization diversity, two dipole pairs 160 may be arranged such that the transverse arms in a given dipole pair are orthogonally arranged with respect to the transverse arms in the remaining dipole pair.
Depending upon the desired operating frequencies, each dipole pair 160 may have multiple transverse arms. The length of each transverse arm is approximately one-fourth of the wavelength for the desired operating frequency. For example, a 2.5 GHz signal has a quarter wavelength of approximately 30 mm, whereas a 10 GHz signal has a quarter wavelength of approximately 7.5 mm. Similarly, a 40 GHz signal has a free-space quarter wavelength of 2.1 mm. Thus, a T-shaped dipole 160 configured for operation at these frequencies would have three transverse arms having fractions of lengths of approximately 30 mm, 7.5 mm and 2.1 mm, respectively. The longitudinal arm of each T-shaped element may be varied in length from 0.01 to 0.99 of the operating frequency wavelength depending upon the desired performance of the resulting antenna. For example, for an operating frequency of 105 GHz, a longitudinal arm may be 500 micrometers in length and a transverse arm may be 900 micrometers in length using a standard semiconductor process. In addition, the length of each longitudinal arm within a dipole pair may be varied with respect to each other. The width of a longitudinal arm may be tapered across its length to lower the input impedance. For example, it may range from 10 micrometers in width at the via end to hundreds of micrometers at the opposite end. The resulting input impedance reduction may range from 800 ohms to less than 50 ohms.
Advantageously, each antenna element 101 is formed using an available metal layer provided by the semiconductor manufacturing process. Each metal layer forming an antenna element may be copper, aluminum, gold, or other suitable metal. To suppress surface waves and block the radiation vertically, insulating layer 105 between adjacent antenna elements within a dipole pair may have a relatively low dielectric constant such as ε=3.9 for silicon dioxide. The dielectric constant of the insulating material beneath the antenna elements forming the remainder of the layers may be relatively high such as ε=7.1 for silicon nitride, ε=11.5 for Ta2O5, or ε=11.7 for silicon. Similarly, the dielectric constant for the insulating layer 105 above ground plane 120 should also be very low (such as ε=3.9 for silicon dioxide, ε=2.2 for Teflon, or 1.0 for air should the insulating layer comprise a honeycombed structure).
The quarter wavelength discussion with respect to each antenna element may be generally applied to other antenna topologies such as patch antennas. However, note that it is only at relatively high frequencies such as the upper bands within the W band of frequencies that the quarter wavelength of a carrier signal in free space is comparable or less than the thickness of substrate 150. Accordingly, at lower frequencies, integrated antennas should be elevated away from the substrate by using an interim dielectric layer. An exemplary beam forming system 200 having such an interim dielectric layer is shown in FIG. 2. Several T-shaped dipole antennas 201 are shown in FIG. 2. A semiconductor substrate 250 includes RF driving circuitry 230 that drives each T-shaped dipole antenna 201 through vias 210 analogously as discussed with respect to beam forming system 100. However, a grounded shield 120 is separated from the T-shaped dipole antennas 201 by a relatively thick dielectric layer 240. For example, dielectric layer 240 may be 1 to 2 mm or more in thickness. In this fashion, lower frequency performance is enhanced. In addition, dielectric layers 240 and an inter-layer dielectric layer 270 may be constructed from flexible materials for a conformal application. Layers 240 and 270 may be separated by an additional ground plane 225.
Although the beam forming systems of FIGS. 1 and 2 advantageously may be integrated onto a semiconductor wafer, the driving transistors are formed on a substrate surface that faces the antennas. As the number of antennas within the array is increased, the coupling of signals to the antenna's driving circuitry becomes cumbersome, particularly for a wafer-scale design.
Accordingly, there is a need in the art for improved wafer scale beam forming antenna systems.